Typically, water from municipal water supplies, must be purified or treated, in order to reduce the level of contaminants present in the water to levels that are acceptable for consumption, or other human use. Contaminants which may be present in municipal tap water include, for example, toxic ionic contaminants, organic compounds, microbes, mold, and/or algae. The required standard of water quality varies depending upon application and may be regulated by various government agencies and trade organizations. Drinking water, for example, must meet the requirements of the National Secondary Drinking Water Regulations, issued by the U.S. Environmental Protection Agency (EPA). The water quality for water used in hemodialysis must meet standards set by the Association for the Advancement of Medical Instrumentation (AAMI) (and subsequently approved by the American National Standards Institute [ANSI]) (ANSI-AAMI water standards). The ANSI-AAMI water standards far exceed the standards required for drinking water. This is because, during dialysis, a large amount of water is almost directly in contact with the patient's blood (separated by only the thin semipermeable membrane of the dialyzer). As such, it is very important that contaminants which are typically present in tap water, be removed prior to use in dialysis.
Sorption processes are generally ideal for removing contaminants from water. Sorbent processes are operationally simple, require virtually no start-up time, and are forgiving toward fluctuations in feed compositions.
Zirconium ion-exchange resins, and particularly, mixed bed ion-exchange resins, can provide useful sorbents for de-ionizing water and removing toxic contaminants in water. Zirconium ion-exchange sorbents are safe and non-toxic. Zirconium resins can remove endotoxins (bacteria) and bacteria does not proliferate in zirconium ion-exchange resins, as they do in organic ion-exchange resins. The zirconium ion-exchange resins are also not vulnerable to attack by chlorine and are thermally stable so that they can be used to treat water, even at high temperatures. Zirconium ion-exchange resins provide a more economical and compact water purification system than conventional water purification systems, such as reverse osmosis (RO) systems, and organic ion-exchange resins. Zirconium ion-exchange resins can efficiently remove toxic inorganic chemicals, especially at high concentrations. Zirconium ion-exchange resins can be configured in a compact form for use in disposable water treatment devices.
In the past, zirconium ion-exchange sorbents which contain ZrP in the Na+ form, for example Na+—ZrP, and HZO in the Cl− form, for example HZO—Cl−, could not be used effectively primarily because of the following factors:    (i) The adsorption capacity and selectivity of ZrP and HZO are affected by the form of counter ions carried by the ion-exchange materials. For example, deionization of water is not possible when ZrP is used in the Na+ form (Na+—ZrP) and HZO is used in the Cl− form (HZO—Cl−) due to the release of Na+ and Cl− ions by the ion-exchange reactions as follows:Na+—ZrP+M+(aq)→M+−ZrP+Na+(aq)  (Reaction I)HZO—Cl−+An−(aq)→HZO−An−+Cl−(aq)  (Reaction II)            Thus, the mixing of ZrP and HZO in this case can, at most, produce the effect of water softening. The loading of the counter ions in these materials further reduces capacity for other ions and limit their use due to the selectivity of ionic adsorption. For example, HZO.Ac (where Ac−=acetate ion) has little adsorption capacity for chloride and nitrate because these anions lie below acetate in the affinity series.            (ii) Deionization of water is affected by leachable PO43− from ZrP and Na+ from HZO; and    (iii) The small particle size of Na+—ZrP and HZO—Cl− results in high resistance to water flow.
Zirconium ion-exchange sorbents for purifying water, which contain only a mixture of acid zirconium phosphate (AZP) and alkaline hydrous zirconium oxide (NaHZO), and no other zirconium ion-exchange resin, also suffer drawbacks. In conventional mixed bed ion-exchange resins, the displacement of previously adsorbed cationic contaminants (e.g. Ca2+, K+, Mg2+) from AZP by excessive hydrogen ions can be a problem due to the weak affinity of the contaminants that can cause a leakage problem. Also, cationic contaminants, with strong sorption affinity can displace each other after adsorption by AZP.
Accordingly, there is a need for a more effective sorbent for selective de-ionizing and removal of contaminants from water. There is also a need for sorbent that can remove the target contaminant without causing significant changes in pH or in the composition of the influent water as caused by the pH change.